1. Field of the Invention
The present invention relates to a semiconductor device which can be manufactured by using, in the same manufacturing process, both the technique of forming a contact plug of the source-drain by a self-alignment method, with respect to a gate electrode, and the technique of metallizing a gate electrode, and to a method of manufacturing such a device.
2. Description of Related Art
In order to downsize the MOS transistor and increase the operation speed at the same time, it is essential to decrease the resistance of the gate electrode. The most popular technique for decreasing the resistance is to use a so-called "polycide gate electrode", in which a silicide layer is formed on a polysilicon layer. In order to further decrease the resistance of the "polycide gate electrode", it is required to increase the height of the gate electrode. However, as the height of the gate electrode is increased, the process for self-aligning a contact such as a drain with respect to a gate electrode, becomes more difficult.
In order to break through the limit of a low resistance of the "polycide gate electrode", there has been conventionally proposed a so-called "polymetal gate electrode", in which, in place of the silicide layer, a metal layer made of tungsten (W layer) or the like is laminated on a polysilicon layer.
However, the "polymetal gate electrode" entails the drawbacks that it is very difficult to perform a post-oxide film forming process for the gate electrode, which is carried out to improve the breakdown voltage of the gate insulation film, and it is difficult to perform a self-alignment process of the contact with respect to the gate electrode.
For example, in the case where silicides 73a and 73b are formed, as can be seen in FIG. 1, on a source-drain layer 71 and a gate electrode 72, respectively, by a self-alignment method, the following problem occurs. That is, since a silicon nitride layer cannot be formed on a polycide-gate electrode, it is very difficult to form a contact plug of the source-drain layer 71 by the self-alignment method with respect to the gate electrode 72.
Further, in the case of a so-called "polymetal-gate electrode" structure in which a metal member 75 such as tungsten is adhered onto the gate electrode of the polysilicon layer 74 via a barrier metal such as of tungsten nitride as can be seen in FIG. 2, so as to decrease the resistance of the gate electrode, there arises an additional problem other than that it is difficult to form a contact plug of the source-drain layer 71 by the self-alignment method with respect to the gate electrode since a silicon nitride layer cannot be formed on the gate electrode. That is, the process for forming a post oxide film of the gate electrode, which is carried out to enhance the breakdown voltage of the gate insulation film, requires a very special oxidization condition.
As described above, in the case where the gate electrode of a transistor is metallized, it does not have a heat resisting property to stand a high-temperature heat treatment step, and it is very difficult to make a contact self-aligned with respect to the gate electrode.